f2fs: introduce f2fs_base_attr for global sysfs entries

[ Upstream commit 21925ede ]

In /sys/fs/f2fs/features, there's no f2fs_sb_info, so let's avoid to get
the pointer.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>